[ffigen] Move core ObjC classes to package:objective_c #121

Job	Run time
analyze_jnigen (stable)	35s
analyze_jni	1m 5s
build_jni_example_android	3m 31s
build_jni_example_linux	1m 17s
run_pdfbox_example_linux	2m 59s
build_jni_example_windows	4m 53s
test_jnigen_windows_minimal	8m 9s
test_jnigen_macos_minimal	4m 42s
test_jnigen (ubuntu-latest, stable)	9m 19s
test_jnigen (ubuntu-latest, beta)	9m 8s
test_jni_windows_minimal	2m 58s
test_jni_macos_minimal	2m 12s
test_jni	1m 56s
coveralls_finish	5s
	52m 49s

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